Thermosetting Adhesives
The emulsion adhesives of the invention comprise a heat curable thermosetting composition which can be used in the form of an aqueous polymer emulsion. The adhesive when cured, in contact with a substrate, can crosslink to form strong thermosetting bonds. Such adhesives can be used in many end uses including in the manufacture of nonwoven fabrics.
Woven fabrics are distinguishable from nonwoven fabrics since woven fabrics obtain mechanical strength and stability from manufacturing operations such as conventional weaving and knitting. Such manufacturing operations result in a generally regular or periodic interaction, interweaving or arrangement of individual fibers, threads, or yarns in a fabric. In sharp contrast, nonwoven fabrics are typically flexible or inflexible sheet-like materials produced directly by adhesively bonding fibers without conventional weaving, knitting or other typical woven fabric manufacturing operations.
Typically nonwoven fabrics are produced by adhesively bonding a loosely assembled collections of fibers which are typically laid in a random, unordered and unarranged fashion. Typically the unbonded irregular arrangement of fibers is not inherently mechanically stable in a fabric. Mechanical strength, stability and integrity of nonwoven fabrics arise from adhesive bonds between fibers.
Nonwoven fabrics are found in many end uses which require a number of unique physical properties. Nonwoven fabrics are desirably strong, heat resistant, solvent resistant, easily made, and must be resistant to a fabric failure in which the nonwoven fibers revert to the loose collection of fibrous starting materials. In order to achieve these properties, the adhesive used in conjunction with nonwoven fibers must bind the fibers into a strong, mechanically stable web or webs, must resist degradation of bond strength as a result of the presence of heat, moisture or solvents, must be easily applied and must rapidly bond the nonwoven fibers.
A number of adhesives have been proposed for nonwoven fabric manufacture including formaldehyde-containing resins, urethane adhesive resins, and acrylic polymeric resin adhesives. The most common nonwoven manufacturing adhesives comprises formaldehyde containing resins which are inexpensive, easy to use and form mechanically stable nonwoven fabrics. However such resins are typically the source of substantial quantities (about 200 to 500 ppm or more in the ambient air) of formaldehyde during curing. Formaldehyde has been identified as a hazardous substance and a great deal of attention has been focused in recent years on a substitute adhesive free of formaldhyde generation. The current limit on formaldehyde concentration in the workplace is about 3 ppm in the ambient air. Further, while the formaldehyde-containing adhesives are generally adequate for most nonwoven fabric manufacture a continued effort has been made to find improved adhesives having properties resulting in improved nonwoven materials.
The prior art has suggested using adhesives such as urethane polymers and acrylic polymers, as is shown in Van Norden Morin, U.S, Pat. No. 2,837,462, Baker, Jr., U.S. Pat. No. 4,207,367, Fulmer et al., U.S. Pat. No. 4,381,332, and others. These alternative adhesives do not appear to have achieved substantial commercial significance.
Clearly a substantial need exists for an improved thermosetting emulsion adhesive that can be used in end uses such as bonding fibers into nonwoven fabrics free of substantial formaldehyde release during cure. The preferred adhesive will provide nonwoven fabrics having high tensile strength, heat resistance, moisture and solvent resistance, and tear resistance.